The objectives of this proposal are to characterize the changes in bile acid kinetics, biliary lipid secretion, and gallbladder physiology that occur during pregnancy and treatment with female sex hormones and may be basic to an increased risk of cholesterol cholelithiasis. Pregnancy and treatment with contraceptive steroids and estrogens increases the risk of this disease. 13C-labeled bile acids will be used for bile acid kinetic studies and methods will be developed to use serum bile acids to facilitate the measurements. The bile acid precursor, 7-alpha-12-alpha-dihydroxy-4-cholesten-3-one, labeled with deuterium, will be employed to investigate in vivo the possible biochemical mechanisms of the disproportionate increase in cholic acid synthesis and pool size recently observed by us in pregnant subjects. Real-time ultrasonography will be employed for the study of gallbladder function. The effects of changes in gallbladder storage and emptying, after various stimuli, on bile acid kinetics and biliary lipid composition will be studied during pregnancy and female sex hormone administration. Methods will be developed for estimating the concentration of bile by the gallbladder and the overnight storage of bile acids. Gallbladder epithelial (Na ion-K ion) ATPase activity will be measured in pregnant and estrogen treated animals. Small intestinal transit time, another regulator of the enterohepatic circulation of bile acids, will also be measured. Non-obese, racially homogenous young women will be studied serially during and after pregnancy, during and after (or before) contraceptive steroid and individual female sex hormone administration. Serum lipid and lipoprotein concentrations, and serum levels of estrogens, progesterone and several peptide hormones will be assayed and correlated with the biliary lipid composition, bile acid kinetics, and gallbladder function.